Visual Factors Perception of the Image

Visual search is the first step in reading a radiograph. At the luminance available for reading an abdominal x-ray, photopic vision is employed. This allows for the greatest visual acuity when the image is focused on the fovea centralis. However, this is a relatively small area owing to the fact that the cones, which are responsible for visual acuity, are concentrated at the fovea, especially at its center, and this accounts for the rapid decline in acuity just a few degrees from the fovea centralis. Of necessity, then, peripheral vision is used as the initial step in reading a radiograph. Use of peripheral vision allows for a considerably larger, although not as acute, field of vision in which to select possible abnormal areas from numerous areas of suboptimal quality images projected on the retina. There is a direct relationship between visual field size and the time required to locate a target.1 After an object of possible interest is located, the eye then moves to a position that focuses the fovea centralis on this point. In this position, detailed information can be obtained.

After an area of interest is identified, the eye moves to another area of interest.2,3 Each movement, known as

Fig. 1—4. Pelvic contents and relationships of an adult female.

Visual Factors: Perception of the Image • 5

Fig. 1—5. Examples ofmul-tistability and figure-ground reversal.

(a) The white area can be viewed as a goblet or the black area as the silhouettes of two profiles. Both images, however, cannot be perceived simultaneously.

(b) The black images can be seen as devils or the white outlines as angels. (Reproduced from M. Escher.)

Fig. 1—5. Examples ofmul-tistability and figure-ground reversal.

(a) The white area can be viewed as a goblet or the black area as the silhouettes of two profiles. Both images, however, cannot be perceived simultaneously.

(b) The black images can be seen as devils or the white outlines as angels. (Reproduced from M. Escher.)

Fig. 1—6. A drawing of a man's face subtly changes to the outline of a young female.

The transition point is dependent not only on subjective variations but on the sequence followed.

Fig. 1—6. A drawing of a man's face subtly changes to the outline of a young female.

The transition point is dependent not only on subjective variations but on the sequence followed.

a saccade, allows for the fovea centralis to fix on a new point of interest. Typically there are two to three saccadic movements per second. The actual visual fixations occupy 90% of viewing time and the saccadic movements 10%. The normal human eye, using photopic vision, can resolve a visual angle of only 2.0°. Translated to reading a 14 x 17 in. film of the abdomen, this means that at 30 in. detailed vision is equal to a circle of 1 in. in diameter, or that 300 separate eye fixations are necessary to cover the entire film.4

Visual fixations tend to cluster around angles and sharp curves.2,3 In addition, if a contour is unpredictable or unusual, that is, it changes direction irregularly and rapidly, visual fixation also will cluster at these points. Wide individual variations in regard to search patterns by radiologists are influenced by prior knowledge.4,5

In addition to topographic changes, other factors such as contrast, size, and shape are important. A target of relatively high contrast will be rapidly detected by peripheral vision. Also, a single high-contrast target that occupies an empty field is detected easily and almost immediately fixated.6 Similar items of the same shape and size take a prolonged time to detect if the contrast is low.7 Size and luminance also have been shown to affect eye fixation.8

It can be concluded that the brain has chosen certain informative details in order to remember or recognize an object. This internal perception within the brain can then take place either by the serial recognition or by the one-step holistic process. Recent evidence, especially in regard to visual perception, tends to support the step-by-step theory. Visual perception and internal perception are not mechanical recordings of elements, but working together are the means for grasping significant structural patterns.9 Examples of this are dramatically illustrated by figure-ground reversal or multistability in perception10(Fig. 1-5). How one image superimposed upon another or projected within another is appreciated visually is determined by which is seen as the figure and which is perceived as the background. The factors that influence the perception of shape in the basic figure-ground relationship of any picture have been extensively studied,11-14 and are directly applicable to analysis of images of the abdomen and pelvis. These laws of visual organization involve area, closedness, symmetry, and continuity. The smaller a closed region, the more an area has closed contours; the more symmetrical a closed region, the more it tends to be seen as a figure. It is often these features that permit the identification of an abnormality discriminated from the background of nor-malcy.15

The point of basic interest in multistable images that spontaneously alternate, as in Figure 1-5, is that one line can have two shapes. A simple curved line is convex on one side but concave on the other. The perceptual representation of a contour is specific to which side is regarded as primary. This achieves added significance when one regards the internal representation or memory of an object as a step-by-step process.

It must also be appreciated, however, that expecta tions that the reader brings to a film may be misleading. This has been demonstrated by psychologists by a series of drawings with subtle progressive differences until the last panel depicts an illustration radically different from the first (Fig. 1-6). The recognizable point of transition where the image shifts in the viewer's perception is different, depending whether the viewer traces the series from left to right or backward. This illustrates that preconditioning—in other words, the concepts of expectation, prior knowledge, and experience—determine in large measure visual perceptions. "Perception depends on learning. It is not that (the radiologist) will see only what he expects to see, but that he will also see what contradicts his expectations."16 The influence of prior knowledge aids the radiologist not only to fixate on important areas of an image, but to search or scan other areas to confirm or refute his or her memory expectation. Distinction is made between perceptual errors, in which image features are not appreciated, and cognitive or reasoning errors, in which the image features, though appreciated, lead to wrong conclusions.17 In wide experiences, perceptual error and underreading (false-negative errors) occur many times more frequently than cognitive error or overreading (false-positive errors).17,18 The limitations to computational theories of perception that integrate work in neurophysiology, psychology, and artificial intelligence19,20 testify to the astonishing complexity and sophistication of human vision.

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